The present invention relates to a cutting tool with improved properties for metal machining having a substrate of cemented carbide and a hard and wear resistant coating on the surface of said substrate. The coating is deposited by Physical Vapor Deposition (PVD). The coating is composed of metal nitrides in combination with alumina (Al2O3). The coating is composed of a laminar multilayered structure. In order to optimize performance, the insert is further treated to have different outer layers on the rake face and flank face, respectively.
Modem high productivity tools for chip forming machining of metals require reliable tools with excellent wear properties. Since the end of 1960s it is known that tool life can be significantly improved by applying a suitable coating to the surface of the tool. The first coatings for wear applications were made by Chemical Vapor Deposition (CVD) and this coating technique is still widely used for cutting tool applications. Physical Vapor Deposition (PVD) was introduced in the mid 1980s and has since then been further developed from single coatings of stable metallic compounds like TiN or Ti(C,N) to include multicomponent and multilayer coatings also including metastable compounds like (Ti,Al)N or non metallic compounds like Al2O3.
RF-sputtering of alumina on cemented carbide cutting tools using deposition temperatures up to 900° C. is described in Shinzato et.al, Thin Sol. Films, 97 (1982) 333-337. The use of PVD coatings of alumina for wear protection is described in Knotek et al, Surf. Coat. Techn., 59 (1993) 14-20, where the alumina is deposited as an outermost layer on a wear resistant carbonitride layer. The alumina layer is said to minimize adhesion wear and acts as a barrier to chemical wear. U.S. Pat. No. 5,879,823 discloses a tool material coated with PVD alumina as one or two out of a layer stack, the non-oxide layers being, e.g., TiAl-containing. The tool may have an outer layer of TiN. The Al2O3 may be of alpha, kappa, theta, gamma or amorphous type. Alumina coated tools where the oxide polymorph is of gamma type with a 400 or 440 texture are disclosed in U.S. Pat. No. 6,210,726. U.S. Pat. No. 5,310,607 discloses PVD deposited alumina with a content of >5% Cr. A hardness of >20 GPa and a crystal structure of alpha phase is found for Cr contents above 20%. No Cr addition gives amorphous alumina with a hardness of 5 GPa.
Most coated tools to-day have a top layer of a goldish TiN to make it easy to differentiate by the naked eye between a used and an unused cutting edge. TiN is not always the preferred top layer especially not in applications where the chip may adhere to the TiN layer. Partial blasting of coatings is disclosed in EP-A-1193328 with the purpose to enable wear detection at the same time as the beneficial properties of the underlying coating are retained. Wear on the rake face is mostly chemical in nature and requires a chemically stable compound whereas wear on the flank face is mostly mechanical in nature and requires a harder and abrasive resistant compound.